Flatworm, a worm with a soft, flat body. Some flatworms live in water, chiefly in the sea, but most are parasites that live in the bodies of other animals. Parasitic flatworms are called flukes and tapeworms. The most common freshwater-dwelling flatworm is the planarian.

The flatworm is the lowest animal with a head, a centralized nervous system, complex reproductive organs, a mesoderm, and a bilaterally symmetrical body. (The mesoderm is the middle layer of embryonic cells, from which muscles and other tissues develop. A bilaterally symmetrical body is one in which the right and left halves are almost identical in shape and size.)

Flatworms belong to the phylum Platyhelminthes. Water-dwelling flatworms belong to the class Turbellaria; flukes, Trematoda; tapeworms, Cestoda.